Hi-Res
VersionThis
map shows areas damaged by the Dec. 16, 1811, magnitude 7.2
earthquake. That earthquake was the first of three major
temblors along the New Madrid fault in 1811 and 1812.

Credit:
Seth Stein, based on results by Susan HoughThe New Madrid fault system
does not behave as earthquake hazard models assume and may be in
the process of shutting down, a new study shows.

A team from Purdue and
Northwestern universities analyzed the fault motion for eight
years using global positioning system measurements and found that
it is much less than expected given the 500- to 1,000-year repeat
cycle for major earthquakes on that fault. The last large
earthquakes in the New Madrid seismic zone were magnitude 7-7.5
events in 1811 and 1812.

Estimating an accurate
earthquake threat for the area, which includes parts of Illinois,
Indiana, Tennessee, Arkansas and Kentucky, is crucial for the
communities potentially affected, said Eric Calais, the Purdue
researcher who led the study.

"Our findings suggest the
steady-state model of quasi-cyclical earthquakes that works well
for faults at the boundaries of tectonic plates, such as the San
Andreas fault, does not apply to the New Madrid fault," said
Calais, who is a professor of earth and atmospheric sciences. "At
plate boundaries, faults move at a rate that is consistent with
the rate of earthquakes so that past events are a reliable guide
to the future. In continents, this does not work. The past is not
necessarily a key to the future, which makes estimating
earthquake hazard particularly difficult."

The team determined that the
ground surrounding the fault system is moving at a rate of less
than 0.2 millimeters per year and there is likely no motion. A
paper detailing the work is published in the current issue of
Science magazine.

Seth Stein, co-author of the
paper, said this surface movement represents energy being stored
that could be released as an earthquake.

"Building up energy for an
earthquake is like saving money for a big purchase," said
Stein, the William Deering Professor of Earth and Planetary
Sciences at Northwestern University. "You put money in over
a long period of time and then spend it all at once and have to
start saving again."

With an earthquake, it is
elastic deformation that must be built up. This can be measured
using GPS through movements on the surface, he said.

"The slower the ground
moves, the longer it takes until the next earthquake, and if it
stops moving, the fault could be shutting down," Stein said.
"We can't tell whether the recent cluster of big earthquakes
in the New Madrid is coming to an end. But the longer the GPS
data keep showing no motion, the more likely it seems."

The U.S. Geological
Survey-funded study used data recorded at nine GPS antennas
mounted in the ground in the earthquake zone.

"GPS technology can
measure movement to the thickness of a fishing line," Stein
said. "Use of GPS to study earthquakes shows the impact a
new technology can have. It lets us see that the world is
different than we thought it was."

In the Midwest there are other
faults that show no activity today but have evidence of
earthquakes occurring within the past 10,000 to 1 million years,
Calais said.

"If other faults in the
central and eastern U.S. have been active recently, geologically
speaking, they could potentially be activated again in the
future," he said. "We need to develop a new paradigm
for how earthquakes happen at faults that are inside continents."

Calais and Stein are exploring
possible explanations for the behavior of faults like the New
Madrid. One possibility is that earthquakes in these areas occur
in clusters and then migrate to a nearby fault.

"There is the possibility
that seismicity migrates with time as earthquakes trigger
earthquakes on nearby faults," Calais said. "Geologists
studying the seismic history of faults have found that there have
been earthquakes on several faults in the central and eastern
U.S. and that they seem to produce bursts of earthquakes and then
turn off."

The team is doing additional
analysis and modeling to study this further.